North Korean hybrid missiles 'could threaten US'

New Scientist is reporting that North Korea may be developing sea-based nuclear weapons:

Ballistic missiles capable of striking the US mainland are being developed by North Korea from decommissioned Soviet hardware, according to a UK military journal.

The report in Jane’s Defence Weekly suggests North Korea has modified technology used in old Soviet submarines to construct both land and sea-based ballistic missile systems capable of carrying nuclear warheads.

A sea-based missile system would enable North Korea to threaten mainland US for the first time. It would also place the nation alongside the US, UK, France, China and Russia in having such a strategic threat.

This news does not bode well for the US. What actions should be taken to stop North Korea?